This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Non-random mixing of lipid bilayer components is relevant when domains of different composition coexist. In particular, so-called lipid rafts seem to play important roles in functions of cell membranes such as signaling, protein transport, endocytosis and adhesion. One aspect of raft formation in cell membranes is whether there is actual lateral separation of lipids into coexisting liquid phases, i.e., liquid-ordered (Lo) and liquid-disordered (La) phases. Detection of coexisting lipid bilayer phases in vesicles has been accomplished by a variety of methods. However, the compositions of the coexisting phases have been exceedingly difficult to determine. A full description of coexisting phases includes their respective compositions, which are provided by the thermodynamic tie-lines. Fluorescence microscopy enables visualization of coexisting lipid phases in giant unilamellar phases, but the composition information is missing. ESR is capable of supplying such information.